U.S. patent number 6,280,865 [Application Number 09/405,685] was granted by the patent office on 2001-08-28 for fuel cell system with hydrogen purification subsystem.
This patent grant is currently assigned to Plug Power Inc.. Invention is credited to Glenn A Eisman, James F. McElroy, Norman Peschke.
United States Patent |
6,280,865 |
Eisman , et al. |
August 28, 2001 |
Fuel cell system with hydrogen purification subsystem
Abstract
The invention relates to a fuel cell system with a hydrogen
purification subsystem. The hydrogen purification subsystem can
concentrate hydrogen from the fuel exhaust for recirculation or
storage. The hydrogen purification subsystem can also concentrate
hydrogen from a fuel supply for input into a fuel cell or for
storage. The hydrogen purification subsystem can also concentrate
hydrogen for quantitative comparison with a second stream
containing hydrogen. The hydrogen purification subsystem can also
charge a hydrogen storage device for system use such as meeting
transient fuel cell load increases.
Inventors: |
Eisman; Glenn A (Niskayuna,
NY), McElroy; James F. (Suffield, CT), Peschke;
Norman (Clifton Park, NY) |
Assignee: |
Plug Power Inc. (Latham,
NY)
|
Family
ID: |
23604774 |
Appl.
No.: |
09/405,685 |
Filed: |
September 24, 1999 |
Current U.S.
Class: |
429/411; 429/53;
429/415; 429/423; 429/482 |
Current CPC
Class: |
H01M
8/0662 (20130101); C01B 3/503 (20130101); H01M
8/04089 (20130101); Y02E 60/50 (20130101); C01B
2203/0405 (20130101) |
Current International
Class: |
H01M
8/06 (20060101); H01M 8/04 (20060101); H01M
008/04 () |
Field of
Search: |
;429/12,13,17,19,22,25,53 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Brouillette; Gabrielle
Assistant Examiner: Yuan; Dah-Wei
Attorney, Agent or Firm: Fish & Richardson P.C.
Claims
What is claimed is:
1. A fuel cell system, comprising:
a fuel cell having a fuel inlet and a fuel exhaust;
a hydrogen purification subsystem including a membrane electrode
assembly, the membrane electrode assembly having an anode side and
an cathode side, the anode side being in fluid connection with the
fuel exhaust of the fuel cell;
the anode side and cathode side of the membrane electrode assembly
each having an electrical connector;
a power source connected to the anode and cathode side electrical
connectors of the membrane electrode assembly, the power source
providing a potential across the connectors.
2. The fuel cell system of claim 1, wherein the membrane electrode
assembly comprises a sulphonated fluorocarbon polymer sandwiched on
either side by a platinum based catalyst layer.
3. The fuel cell system of claim 1, wherein the membrane electrode
assembly comprises a PEM fuel cell membrane electrode assembly.
4. The fuel cell system of claim 1, wherein the membrane electrode
assembly anode side is connected to the fuel exhaust of the fuel
cell.
5. The fuel cell system of claim 1, further comprising a fuel
supply system, wherein the membrane electrode assembly anode side
is in fluid connection to the fuel supply system.
6. The fuel cell system of claim 5, wherein the fuel supply system
comprises a reformer.
7. The fuel cell system of claim 5, wherein the fuel supply system
comprises a fuel gas supply line.
8. The fuel cell system of claim 4, further comprising a hydrogen
storage device in fluid connection with the membrane electrode
assembly cathode side.
9. The fuel cell system of claim 8, wherein the hydrogen storage
device comprises a pressure vessel.
10. The fuel cell system of claim 5, further comprising a hydrogen
storage device in fluid connection with the membrane electrode
assembly cathode side.
11. The fuel cell system of claim 10, wherein the hydrogen storage
device comprises a pressure vessel.
12. The fuel cell system of claim 8, wherein the hydrogen storage
device is in fluid connection with the fuel inlet of the fuel
cell.
13. The fuel cell system of claim 10, wherein the hydrogen storage
device is in fluid connection with the fuel inlet of the fuel
cell.
14. The fuel cell system of claim 12, further comprising a valve
system having a first operational state wherein a first flow path
connects the membrane electrode assembly cathode side to the
hydrogen storage device, and a second operational state wherein a
second flow path connects the membrane electrode assembly cathode
side to the fuel inlet of the fuel cell.
15. The fuel cell system of claim 13, further comprising a valve
system having a first operational state wherein a first flow path
connects the membrane electrode assembly cathode side to the
hydrogen storage device, and a second operational state wherein a
second flow path connects the membrane electrode assembly cathode
side to the fuel inlet of the fuel cell.
16. The fuel cell system of claim 14, further comprising a
controller and a hydrogen storage device pressure sensor connected
to the controller, the controller being adapted to actuate the
valve system between the first and second operational states in
response to a signal from the hydrogen storage device pressure
sensor.
17. The fuel cell system of claim 16, further comprising a
transient load sensor connected to the fuel cell, wherein the
controller is connected to the transient load sensor and adapted to
release a stored flow from the hydrogen storage device to the fuel
inlet of the fuel cell in response to a signal from the transient
load sensor.
18. The fuel cell system of claim 17, wherein the transient load
sensor is a hydrogen concentration sensor.
19. A fuel cell system comprising:
a fuel cell having a fuel inlet and a fuel exhaust;
a hydrogen storage device;
a membrane electrode assembly having an anode side and an cathode
side, the cathode side being in fluid connection with the hydrogen
storage device, the anode side having an electrical connector and
the cathode side having an electrical connector;
a power source connected to the anode and cathode side electrical
connectors, the power source providing a potential across the
connectors.
20. The fuel cell system of claim 19, wherein the hydrogen storage
device comprises a pressure vessel.
21. The fuel cell system of claim 19, wherein the membrane
electrode assembly anode side is connected to the fuel exhaust of
the fuel cell.
22. The fuel cell system of claim 19, further comprising a fuel
supply system in fluid connection to the membrane electrode
assembly.
23. The fuel cell system of claim 22, wherein the fuel supply
system comprises a reformer.
24. A fuel cell system comprising:
a fuel cell having a fuel inlet and a fuel exhaust;
a first membrane electrode assembly having an anode side and an
cathode side;
a comparison membrane electrode assembly having a concentrated side
and a comparison side, the concentrated side being in fluid
connection with the cathode side of the first membrane electrode
assembly, the comparison side being connected to a fuel gas
source;
the anode side and cathode side of the first membrane electrode
assembly each having an electrical connector;
a power source connected to the anode and cathode side electrical
connectors, the power source providing a potential across the
connectors;
the concentrated side and comparison side of the comparison
membrane electrode assembly each having an electrical connector,
the connectors each being connected to a voltage measuring
device.
25. The fuel cell system of claim 24, wherein the fuel source
comprises a fuel cell exhaust of the fuel cell.
26. The fuel cell system of claim 24, wherein the fuel source
comprises a fuel processing system output stream.
27. A method of manipulating hydrogen in a fuel cell system,
comprising:
flowing a fuel exhaust of a fuel cell across an anode side of a
hydrogen purification subsystem membrane electrode assembly;
and
applying a potential across the membrane electrode assembly
sufficient to induce electrochemical hydrogen pumping through the
membrane electrode assembly.
28. The method of claim 27, further comprising flowing a
concentrated hydrogen stream from a cathode side of the membrane
electrode assembly to a fuel inlet of a fuel cell.
29. The method of claim 27, further comprising flowing a
concentrated hydrogen stream from a cathode side of the membrane
electrode assembly to a hydrogen storage device.
30. A method of manipulating hydrogen in a fuel cell system,
comprising:
flowing a fuel gas in a fuel cell system across an anode side of a
hydrogen purifification subsystem membrane electrode assembly;
applying a potential across the membrane electrode assembly
sufficient to induce electrochemical hydrogen pumping through the
membrane electrode assembly; and
flowing a concentrated hydrogen stream from a cathode side of the
membrane electrode assembly to a hydrogen storage device.
Description
The invention relates generally to a fuel cell system with a
hydrogen purification subsystem.
BACKGROUND OF THE INVENTION
A fuel cell can convert chemical energy to electrical energy by
promoting a chemical reaction between two reactant gases.
One type of fuel cell includes a cathode flow field plate, an anode
flow field plate, a membrane electrode assembly disposed between
the cathode flow field plate and the anode flow field plate, and
two gas diffusion layers disposed between the cathode flow field
plate and the anode flow field plate. A fuel cell can also include
one or more coolant flow field plates disposed adjacent the
exterior of the anode flow field plate and/or the exterior of the
cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and
open-faced channels connecting the inlet region to the outlet
region and providing a way for distributing the reactant gases to
the membrane electrode assembly.
The membrane electrode assembly usually includes a solid
electrolyte (e.g., a proton exchange membrane, commonly abbreviated
as a PEM) between a first catalyst and a second catalyst. One gas
diffusion layer is between the first catalyst and the anode flow
field plate, and the other gas diffusion layer is between the
second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the reactant gases (the
anode reactant gas) enters the anode flow field plate at the inlet
region of the anode flow field plate and flows through the channels
of the anode flow field plate toward the outlet region of the anode
flow field plate. The other reactant gas (the cathode reactant gas)
enters the cathode flow field plate at the inlet region of the
cathode flow field plate and flows through the channels of the
cathode flow field plate toward the cathode flow field plate outlet
region.
As the anode reactant gas flows through the channels of the anode
flow field plate, the anode reactant gas passes through the anode
gas diffusion layer and interacts with the anode catalyst.
Similarly, as the cathode reactant gas flows through the channels
of the cathode flow field plate, the cathode reactant gas passes
through the cathode gas diffusion layer and interacts with the
cathode catalyst.
The anode catalyst interacts with the anode reactant gas to
catalyze the conversion of the anode reactant gas to reaction
intermediates. The reaction intermediates include ions and
electrons. The cathode catalyst interacts with the cathode reactant
gas and the reaction intermediates to catalyze the conversion of
the cathode reactant gas to the chemical product of the fuel cell
reaction.
The chemical product of the fuel cell reaction flows through a gas
diffusion layer to the channels of a flow field plate (e.g., the
cathode flow field plate). The chemical product then flows along
the channels of the flow field plate toward the outlet region of
the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and
reactant gases from one side of the membrane electrode assembly to
the other side of the membrane electrode assembly. However, the
electrolyte allows ionic reaction intermediates to flow from the
anode side of the membrane electrode assembly to the cathode side
of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode
side of the membrane electrode assembly to the cathode side of the
membrane electrode assembly without exiting the fuel cell. In
contrast, the electrons flow from the anode side of the membrane
electrode assembly to the cathode side of the membrane electrode
assembly by electrically connecting an external load between the
anode flow field plate and the cathode flow field plate. The
external load allows the electrons to flow from the anode side of
the membrane electrode assembly, through the anode flow field
plate, through the load and to the cathode flow field plate.
Because electrons are formed at the anode side of the membrane
electrode assembly, that means the anode reactant gas undergoes
oxidation during the fuel cell reaction. Because electrons are
consumed at the cathode side of the membrane electrode assembly,
that means the cathode reactant gas undergoes reduction during the
fuel cell reaction.
For example, when hydrogen and oxygen are the reactant gases used
in a fuel cell, the hydrogen flows through the anode flow field
plate and undergoes oxidation. The oxygen flows through the cathode
flow field plate and undergoes reduction. The specific reactions
that occur in the fuel cell are represented in equations 1-3.
As shown in equation 1, the hydrogen forms protons (H.sup.+) and
electrons. The protons flow through the electrolyte to the cathode
side of the membrane electrode assembly, and the electrons flow
from the anode side of the membrane electrode assembly to the
cathode side of the membrane electrode assembly through the
external load. As shown in equation 2, the electrons and protons
react with the oxygen to form water. Equation 3 shows the overall
fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction
produces heat. One or more coolant flow field plates are typically
used to conduct the heat away from the fuel cell and prevent it
from overheating.
Each coolant flow field plate has an inlet region, an outlet region
and channels that provide fluid communication between the coolant
flow field plate inlet region and the coolant flow field plate
outlet region. A coolant (e.g., liquid de-ionized water) at a
relatively low temperature enters the coolant flow field plate at
the inlet region, flows through the channels of the coolant flow
field plate toward the outlet region of the coolant flow field
plate, and exits the coolant flow field plate at the outlet region
of the coolant flow field plate. As the coolant flows through the
channels of the coolant flow field plate, the coolant absorbs heat
formed in the fuel cell. When the coolant exits the coolant flow
field plate, the heat absorbed by the coolant is removed from the
fuel cell.
To increase the electrical energy available, a plurality of fuel
cells can be arranged in series to form a fuel cell stack. In a
fuel cell stack, one side of a flow field plate functions as the
anode flow field plate for one fuel cell while the opposite side of
the flow field plate functions as the cathode flow field plate in
another fuel cell. This arrangement may be referred to as a bipolar
plate. The stack may also include monopolar plates such as, for
example, an anode coolant flow field plate having one side that
serves as an anode flow field plate and another side that serves as
a coolant flow field plate. As an example, the open-faced coolant
channels of an anode coolant flow field plate and a cathode coolant
flow field plate may be mated to form collective coolant channels
to cool the adjacent flow field plates forming fuel cells.
Typically only a portion of the fuel (e.g., reformate containing
hydrogen) flowing through a fuel cell will react, so that the fuel
gas exhaust from a fuel cell will generally contain some level of
hydrogen. For example, the amount of hydrogen that is reacted may
depend on factors including temperature, pressure, residence time,
and catalyst surface area. For this reason, excess hydrogen may be
sometimes fed to a fuel cell in order to increase the amount of
reacting hydrogen to a level corresponding to a desired power
output of the fuel cell. For example, it may be that 100 standard
liters per minute (slm) of hydrogen must be reacted in a fuel cell
to achieve a desired power output, but it is determined that 140
slm of hydrogen must be fed to the fuel cell to achieve this
reaction of 100 slm of hydrogen. This system may be said to be
running at 40% excess hydrogen at the anode inlet. In other
terminology, this system may also be characterized as running at a
stoichiometry of 1.4. For similar reasons, it may be desirable to
supply the cathode side of the fuel cell with an excess of oxidant
(e.g., air).
SUMMARY OF THE INVENTION
The invention relates to a fuel cell system with a hydrogen
purification subsystem.
In one embodiment, the hydrogen purification subsystem removes
hydrogen from the fuel gas output stream and transfers it to the
fuel gas input stream. This increases the fuel cell system
efficiency and decreases the amount of hydrogen wasted during use
of the fuel cell system relative to an otherwise substantially
identical fuel cell system having a design in which hydrogen
contained in the fuel gas output stream is simply discarded.
In another embodiment, the hydrogen purification subsystem includes
a membrane electrode assembly (MEA) that is distinct from the MEA's
of the fuel cells in the fuel cell system. The hydrogen pumping MEA
(HPMEA) has an anode side (the side from which hydrogen is pumped)
and a cathode side (the side to which hydrogen is pumped). The
cathode side is connected to the fuel inlet stream of the fuel cell
system. The anode and cathode sides of the HPMEA each have an
electrical connector in contact with a power source that provides a
potential across the HPMEA. The polarity of the potential is
positive on the anode side of the HPMEA and negative on the cathode
side of the HPMEA. The membrane of the HPMEA can be a proton
exchange membrane. It will be appreciated that the term "HPMEA" is
used only to note the application of such an MEA for hydrogen
pumping, and not to indicate its physical characteristics.
In one aspect, the HPMEA anode side can be connected to a fuel
exhaust stream of a fuel cell. The HPMEA anode side can also be
connected to a fuel supply system. The fuel supply system can be,
as examples, a reformer or a fuel gas supply line.
In another aspect, the HPMEA cathode side can be connected to a
hydrogen storage device. The hydrogen storage device can include,
as examples, pressure vessels and other known hydrogen storage
systems such as hydrogen storage alloys.
In another aspect, the hydrogen storage device can be connected to
the fuel inlet stream of a fuel cell. The invention may also
include a valve system having a first operational state wherein a
concentrated hydrogen stream is flowed from the HPMEA cathode side
to a hydrogen storage device where the concentrated hydrogen stream
is isolated from the fuel inlet stream of the fuel cell. The valve
system may also have a second operational state wherein the
concentrated hydrogen stream is flowed from the HPMEA cathode side
to the fuel inlet stream of the fuel cell, where the concentrated
hydrogen stream is isolated from the hydrogen storage device. In
another aspect, the hydrogen purification subsystem may include a
controller connected to a hydrogen storage device pressure sensor.
The controller can be adapted to actuate the valve system between
the first and second operational states in response to a signal
from the hydrogen storage device pressure sensor. In another
aspect, the hydrogen purification subsystem can include a transient
load sensor connected to the fuel cell, wherein a controller is
connected to the transient load sensor and adapted to release a
stored flow from the hydrogen storage device to the fuel inlet
stream of the fuel cell in response to a signal from the transient
load sensor. The transient load sensor can be, for example, a
hydrogen concentration sensor. Other sensors are possible.
In another embodiment, the hydrogen purification subsystem includes
an HPMEA having its cathode side connected solely to a hydrogen
storage device. In one aspect, the HPMEA may have its anode side
connected to a fuel exhaust stream of a fuel cell. In another
aspect, the HPMEA may have its anode side connected to a fuel
supply system of a fuel cell.
In another embodiment, as discussed herein, the HPMEA may be
coupled with a comparison MEA having a concentrated side and a
comparison side. The concentrated side may be connected to the
cathode side of the HPMEA, and the comparison side may be connected
to a fuel gas source. An electric potential may be applied across
the HPMEA to induce hydrogen pumping, and a voltage measuring
device may be used to measure the potential across the comparison
MEA caused by the differential of hydrogen concentration on either
side of the comparison MEA. The voltage may be correlated to the
hydrogen concentration of the fuel gas source. The fuel gas source
may be, as examples, an exhaust stream of a fuel cell, a reformer
outlet stream, a fuel supply line, or in general a fuel gas line
within a fuel cell system.
In another embodiment, the invention provides a method of
manipulating hydrogen in a fuel cell system. In one aspect, a fuel
exhaust stream of a fuel cell is flowed against a first side of an
HPMEA. A potential may be applied across the HPMEA that is
sufficient to induce electrochemical hydrogen pumping through the
HPMEA. The resulting concentrated hydrogen stream may be flowed, as
examples, to a fuel inlet stream of a fuel cell or to a hydrogen
storage device.
Multiple hydrogen purification subsystems can be used within a
single fuel cell system, for example, to concentrate fuel inlet gas
while also concentrating a recirculated portion of the fuel exhaust
gas.
Other advantages and features will become apparent from the
following description of the preferred embodiments and the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a fuel cell system with a hydrogen
purification subsystem according to an embodiment of the
invention;
FIG. 2 is a schematic diagram of a fuel cell system with a hydrogen
purification subsystem according to another embodiment of the
invention;
FIG. 3 is a cross-sectional view of a hydrogen pumping system
according to an embodiment of the invention;
FIG. 4 is a schematic diagram of a portion of the hydrogen
purification subsystem of FIG. 1.
FIG. 5 is a schematic diagram of a fuel cell system with a hydrogen
purification subsystem according to another embodiment of the
invention;
FIG. 6 is a cross-sectional view of a fuel cell according to an
embodiment of the invention;
FIG. 7 is an elevational view of a cathode flow field plate
according to an embodiment of the invention;
FIG. 8 is an elevational view of an anode flow field plate
according to an embodiment of the invention; and
FIG. 9 is an elevational view of a coolant flow field plate
according to an embodiment of the invention.
DETAILED DESCRIPTION
Referring to FIG. 1, a schematic diagram is shown of a fuel cell
system 2 with a hydrogen purification subsystem 4 according to an
embodiment of the invention. Fuel cell stack 6 is shown indicating
the direction of fuel gas 7 through the stack 6. Fuel gas 7 flows
from fuel inlet line 5 through fuel gas inlet 8 into fuel inlet
manifold 10. The fuel gas 7 travels through individual fuel cells
12 (see FIGS. 6-9 and accompanying discussion) and into fuel
exhaust manifold 14. The fuel gas exhaust 15 exits the stack 6
through fuel gas outlet 16.
Fuel exhaust line 18 is circulated through device 34 (see
discussion below) and connected to effluent gas recirculation (EGR)
line 19. EGR fan 20 is located on line 19 to drive the
recirculation. EGR line may also include a check valve (not shown)
to prevent back flow from fuel inlet line 5 into fuel exhaust line
18. As an example, the inlet fuel gas 7 may initially contain about
40% hydrogen, and due to reaction of hydrogen in the fuel cell
stack 6, the fuel gas exhaust 15 may contain about 11% hydrogen. It
may be desirable to recirculate about 50% of the fuel gas exhaust
15. The rest could be sent to vent 22. Fuel exhaust line 18 is
further connected to hydrogen pumping device 24. Hydrogen pumping
device 24 is connected to power source 26 which provides an
electric potential across the device 24. Hydrogen pumping device 24
has vent 43 to vent the fuel exhaust gas after its hydrogen is
removed by device 24.
On the side of the hydrogen pumping device 24 opposite from the
fuel exhaust line 18, the hydrogen pumping device 24 is connected
to recirculation valve 23, which can be adjusted to select a
portion of the hydrogen pumping device effluent 28 for
recirculation into fuel inlet line 5. As discussed herein, the
hydrogen pumping device effluent 28 may be substantially pure
hydrogen. The hydrogen pumping device 24 is also connected to
hydrogen storage device 30. Hydrogen storage device 30 may be a
pressure vessel, or other hydrogen storage systems known in the
art, such as a hydrogen storage alloy system. Valve 32 controls the
flow to and from the hydrogen storage device 30. During normal
operation, valve 32 may be closed and valve 23 may be opened. In
this state, the hydrogen purification subsystem 4 serves to
recirculate pure hydrogen from the fuel gas exhaust 15.
The subsystem 4 can also be used to charge the hydrogen storage
device 30. For example, when valve 23 is closed and valve 32 is
open, the hydrogen pumping device 24 can pressurize hydrogen
storage device 30 (see discussion with respect to FIG. 5). Once
pressurized, valve 32 can be shut to store the hydrogen. In one
embodiment, in response to a transient load increase on the fuel
cell system 2, valves 32 and 23 can be opened to provide a rapid
increase in the amount of hydrogen available to the fuel cell stack
6. As an example, this may be advantageous if the fuel gas supply
is not capable of rapidly increasing the available hydrogen. For
example, where a reformer is used to provide a reformate fuel gas,
depending on the design, it may be the case that there is a lag
time before the available hydrogen from the reformer can be
increased in response to a signal indicating a transient load
increase. For example, it may take from 5 to 20 seconds for the
reformer to reach a steady increased fuel output in response to a
transient load increase.
In the embodiment shown in FIG. 1, the hydrogen pumping device
effluent 28 is further connected to the concentrated side 36 of a
comparison device 34. This connection may have a valve (not shown)
to isolate comparison device 34 when storage device 30 is being
pressurized. Comparison device 34 may include an MEA similar to
hydrogen pumping device 24 (see FIG. 4). Comparison device 34 has a
vent 41 to periodically purge the concentrated hydrogen stream
between devices 24 and 34. Comparison side 38 of the comparison
device 34 is connected to the fuel gas exhaust 15 from the stack 6.
As discussed with respect to FIG. 4, voltage measuring device 40
measures the potential across fuel cell 34 that arises from the
difference in hydrogen concentration on either side of the
comparison device 34. The voltage measured by device 40 can be
correlated to determine the partial pressure of hydrogen in fuel
gas exhaust 15, which indicates the concentration of hydrogen in
fuel gas exhaust 15. For example, the Nernst equation may be used
to calculate the partial pressure of hydrogen on the fuel gas
exhaust 15 side of the comparison device 40:
E--measured voltage;
E.sub.0 --reactant equilibrium potential
R--universal gas constant;
T--temperature;
n--number of electrons transferred;
F--Faraday constant;
P.sub.1 --hydrogen partial pressure of hydrogen pumping device
effluent; and
P.sub.2 --partial pressure of hydrogen in the fuel gas exhaust
stream.
As an example, a decrease in the hydrogen concentration of fuel gas
exhaust 15 may indicate a transient load increase. This indication
can be used, for example, to open hydrogen storage device 30 as
previously discussed to provide increased hydrogen fuel in response
to the load increase.
Controller 42 is shown connected to various components of subsystem
4. However, it will be appreciated that the present invention is
not limited by any particular control scheme. It will further be
appreciated that the schematic diagram shown in FIG. 1 is for
illustrative purposes only, and does not limit the scope of the
invention to a particular embodiment.
Referring to FIG. 2, a schematic diagram is shown of a fuel cell
system 50 with a hydrogen purification subsystem 52 according to an
embodiment of the invention that does not include the comparison
device 34. For example, the subsystem 52 may be actuated to utilize
hydrogen storage device 30 in response to a transient load
increase, where the load increase is measured by some other method,
such as electronically. In other possible embodiments, the hydrogen
purification subsystem of the invention may not include a hydrogen
storage device. For example, the system may be used simply to
provide a recirculated stream of pure hydrogen as previously
discussed. In the embodiment shown in FIG. 2, the fuel inlet gas 7
is supplied by reformer 35. For example, reformer 35 may be used to
convert a hydrocarbon fuel such natural gas or methanol into
hydrogen, as known in the art.
FIG. 3 shows an embodiment of a hydrogen pumping device 60.
Hydrogen pumping device 60 includes a first flow field plate 62, a
second flow field plate 64, an electrolyte 66, catalysts 68 and 70
and gas diffusion layers 72 and 74. Electrolyte 66 should be
capable of allowing ions to flow therethrough while providing a
substantial resistance to the flow of electrons. Electrolyte 66 is
a solid polymer (e.g., a solid polymer ion exchange membrane), such
as a solid polymer proton exchange membrane (e.g., a solid polymer
containing sulfonic acid groups). Such membranes are commercially
available from E.I. DuPont de Nemours Company (Wilmington, Del.)
under the trademark NAFION. Alternatively, electrolyte 66 can also
be prepared from the commercial product GORE-SELECT, available from
W.L. Gore & Associates (Elkton, Md.).
Catalysts 68 and 70 can be formed of a material capable of
interacting with hydrogen to form protons and electrons. Examples
of such materials include, for example, platinum, platinum alloys,
and platinum dispersed on carbon black. Catalyst layers 68 and 70
may be formed onto electrolyte 66. Alternatively, catalyst layers
68 and 70 may be applied to the surfaces of gas diffusion layers 72
and 74.
Gas diffusion layers 72 and 74 may be formed of a material that is
both gas and liquid permeable material so that the fuel gas and any
water condensing from the fuel gas or entrained therein can pass
through the gas diffusion layers 72 and 74. Layers 72 and 74 should
be electrically conductive so that electrons can flow from
catalysts 68 and 70 to flow field plates 62 and 64, respectively.
In some embodiments, the gas diffusion layers maybe omitted. In
such cases, a power source (not shown) may be connected directly to
either side of the membrane electrode assembly.
As previously discussed, an MEA refers to the sandwich of the
electrolyte 66 within the catalyst layers 68 and 70. An MEA may be
used with or without gas diffusion layers 72 and 74. Also, it will
be appreciated that flow plates 62 and 64 are also not required
features of a hydrogen pumping device. Other configurations are
possible.
Referring to FIG. 4, a schematic diagram is shown of a portion of
the hydrogen purification subsystem of FIG. 1. Fuel gas exhaust 15
is brought into contact with hydrogen pumping device 24 which
includes an MEA. In the example shown in FIG. 4, hydrogen pumping
device 24 includes electrolyte 76, electrode layers 78, and gas
diffusion layers 80. Power source 26 applies a potential across
device 24, inducing the following reaction of the hydrogen in the
fuel gas exhaust 15 as it contacts catalyst layer 78:
The protons from the reaction flow through the electrolyte 76, and
the electrons flow around the MEA 24 to re-form hydrogen according
to the following reaction:
Effluent 28 from hydrogen pumping device 24 (FIG. 1) is
substantially pure hydrogen because other components of fuel
exhaust gas 15 are not passed through the MEA 24. Thus, a reference
stream 82 of pure hydrogen is formed.
The amount of hydrogen transported through MEA 24 depends on the
amount of current supplied by power source 26. Referring to the
direction of hydrogen flow, the MEA 24 has an anode side 27 and a
cathode side 29. The hydrogen reference stream 82 is connected to
comparison membrane electrode assembly 34.
In the example shown in FIG. 4, MEA 34 includes electrolyte 84,
electrode layers 86, and gas diffusion layers 88. The side of MEA
34 opposite from hydrogen reference stream 82 is connected to fuel
gas exhaust 15. Referring to the direction of hydrogen flow, the
MEA 24 has an concentrated side 31 and a comparison side 33. In
this system, a potential is developed across MEA 34 due to the
difference in hydrogen concentration between pure hydrogen
reference stream 82 and fuel gas exhaust 15. Voltage measuring
device 40 measures the potential across MEA 34 and computer 90
correlates this measurement into a hydrogen concentration
measurement of the fuel gas exhaust 15.
The structure of the example shown in FIG. 4 is simplified. It will
be appreciated that the hydrogen reference stream 82 may be
isolated from other gasses in the fuel cell system to maintain its
purity. It will also be appreciated that various valve and piping
configurations may be implemented to accommodate various
objectives, such as charging the hydrogen storage device 30 as
previously described.
Referring to FIG. 5, a schematic diagram is shown of a fuel cell
system 92 with a hydrogen purification subsystem 94 according to
another embodiment of the invention. Fuel cell stack 96 has fuel
inlet stream 98 and fuel outlet stream 100. A first portion of fuel
outlet stream is recirculated into fuel inlet stream 98 through
first recirculation stream 102. A second portion of fuel outlet
stream is flowed in a second recirculation stream 104 to hydrogen
purification subsystem 94. Subsystem 94 has an subsystem inlet 106,
a subsystem outlet 108, and a vent 110. Vent 110 disposes of what
remains of second recirculation stream 104 after it has passed
through subsystem 94. Subsystem 94 has at least one power supplying
fuel cell 112 and at least one hydrogen pumping device 114.
Subsystem 94 may also have an activation switch 116 connected to
electrical connectors 118 and 120. While power supplying fuel cell
112 is part of the fuel cell stack 96, it is electrically separated
by electrical connector 118. In other words, when fuel cell stack
96 is in operation and switch 116 is closed, the power supplying
fuel cell 112 generates a voltage potential across electrical
connectors 118 and 120. In this way, a potential is provided across
hydrogen pumping device 114 to induce hydrogen pumping. Where
switch 116 is opened, the second recirculation stream 104 passes
through subsystem 94 and out vent 110 without having hydrogen in
stream 104 removed by the hydrogen pumping device.
For example, a voltage of 0.5 VDC across fuel cell 112 may result
in about 7.5 cubic centimeters of hydrogen being "pumped" through
hydrogen pumping device 114 for each amp of current flow.
Subsystem effluent stream 120 is connected to hydrogen storage
device 122 and to the fuel inlet stream 98 of the stack 96. The
subsystem effluent stream 120 and hydrogen storage device 122 may
have valve configurations (not shown), for example, as discussed
with respect to FIG. 1. Hydrogen storage device 122 may be, for
example, a pressure vessel. Where it is desired to charge the
pressure of hydrogen storage device 122, the current supplied to
fuel cell 112 may be selected to produce a sufficient amount of
hydrogen to result in the desired pressure. For example, it may be
desirable for a pressure vessel hydrogen storage device 122 to have
about 1 cubic foot of storage volume, and be charged at about 2
atmospheres of pressure.
FIG. 6 shows an embodiment of a fuel cell 200 designed to catalyze
the fuel cell reaction. Fuel cell 200 includes a cathode flow field
plate 210, an anode flow field plate 220, a solid electrolyte 230,
catalysts 240 and 250 and gas diffusion layers 260 and 270.
Electrolyte 230 should be capable of allowing ions to flow
therethrough while providing a substantial resistance to the flow
of electrons. Electrolyte 230 is a solid polymer (e.g., a solid
polymer ion exchange membrane), such as a solid polymer proton
exchange membrane (e.g., a solid polymer containing sulfonic acid
groups). Such membranes are commercially available from E.I. DuPont
de Nemours Company (Wilmington, Del.) under the trademark NAFION.
Alternatively, electrolyte 230 can also be prepared from the
commercial product GORE-SELECT, available from W.L. Gore &
Associates (Elkton, Md.).
Catalyst 240 can be formed of a material capable of interacting
with hydrogen to form protons and electrons. Examples of such
materials include, for example, platinum, platinum alloys, and
platinum dispersed on carbon black. Alternatively, the suspension
is applied to the surfaces of gas diffusion layers 260 and 270 that
face catalysts 240 and 250, respectively, and the suspension is
then dried. The method of preparing catalyst 240 may further
include the use of heat, pressure and temperature to achieve
bonding.
Catalyst 250 can be formed of a material capable of interacting
with oxygen, electrons and protons to form water. Examples of such
materials include, for example, platinum, platinum alloys, and
noble metals dispersed on carbon black. Catalyst 250 can be
prepared as described above with respect to catalyst 240.
Gas diffusion layers 260 and 270 are formed of a material that is
both gas and liquid permeable material so that the reactant gases
(e.g., hydrogen and oxygen) and products (e.g., water) can pass
therethrough. In addition, layers 260 and 270 should be
electrically conductive so that electrons can flow from catalysts
240 and 250 to flow field plates 220 and 210, respectively.
FIG. 7 shows an embodiment of cathode flow field plate 210 which is
used to provide a flow path that allows the oxygen to interact with
catalyst 250. Cathode 210 has an inlet 212, an outlet 214 and
open-faced channels 216 that define a flow path for an oxidant gas
from inlet 212 to outlet 214. An oxidant gas input stream (not
shown) flows to inlet 212. As the oxidant gas flows along channels
216, the oxygen contained in the oxidant gas permeates gas
diffusion layer 270 to interact with catalyst 250, electrons and
protons to form water. The water can pass back through diffusion
layer 270, enter the oxidant stream in channels 216, and exit fuel
cell 200 through cathode flow field plate outlet 214.
FIG. 8 shows an embodiment of anode flow field plate 220 which is
designed to provide a flow path for a fuel gas that allows the
hydrogen to interact with catalyst 24. Cathode flow field plate 220
has an inlet 222, outlet 224 and open-faced channels 226 that
define a flow path for a fuel gas from inlet 222 to outlet 224. A
fuel gas input stream (not shown) flows to inlet 222. As the fuel
flows along channels 226, the hydrogen contained in the fuel gas
permeates gas diffusion layer 260 to interact with catalyst 240 to
form protons and electrons. The protons pass through solid
electrolyte 230, and the electrons pass are conducted through gas
diffusion layer 260 to anode flow field plate 220, ultimately
flowing through an external load to cathode flow field plate
210.
The heat produced during the fuel cell reaction is removed from
fuel cell 200 by flowing a coolant through the fuel cell via a
coolant flow field plate. FIG. 9 shows an embodiment of coolant
flow field plate 530 having an inlet 532, an outlet 534 and
open-faced channels 536 that define a flow path for coolant from
inlet 532 to outlet 534. The coolant enters fuel cell 200 from a
coolant input stream via inlet 532, flows along channels 536 and
absorbs heat, and exits fuel cell 200 via outlet 534 to a coolant
output stream (not shown). The coolant enters fuel cell 200 from
coolant input stream 800 via inlet 532, flows along channels 536
and absorbs heat, and exits fuel cell 200 via outlet 534 to a
coolant output stream (not shown).
Although certain embodiments and arrangements of cathode flow field
plate 210, anode flow field plate 220 and coolant flow field plate
530 have been described herein, other embodiments and arrangements
of these flow field plates can also be used. For example, other
embodiments are disclosed in commonly assigned U.S. patent
application Ser. No. 09/168,232, entitled "Fuel Cell Assembly Unit
for Promoting Fluid Service and Design Flexibility", which is
hereby incorporated by reference.
Moreover, while a fuel cell system containing a single fuel cell
has been described herein, the fuel cell system is not limited to
such single cell embodiments. Rather, the fuel cell system can
include a plurality of fuel cells. Typically, the fuel cells in
such systems are arranged in series by having the back surface of a
cathode flow field plate in one fuel cell serve as the anode flow
field plate in the next fuel cell in the series arrangement. A
plurality of coolant flow field plates can also be used in these
systems. Examples of fuel cell systems having a plurality of fuel
cells and coolant flow field plates are described in U.S. patent
application Ser. No. 09/168,232.
While certain embodiments of the invention, as well as their
principals of operation, have been disclosed herein, the invention
is not limited to these embodiments or these principals of
operation. Other embodiments are in the claims.
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